Ecology I

Community Ecology

Questions asked by Community EcologistsHow do species coexist?How do they relate to each other?How do communities change with time?

Food chains and webs

Food chain = simplified linear diagram of who eats whom

Food web = complex network of who eats whom

We can represent feeding interactions (and thus energy transfer) in a community:

Classification based on trophic level Trophic level: all organisms that share the same general

type of food.

Unit 2: Ecology 1: Principles of Matter, Energy and LIfe 4

Aquatic examples Terrestrial examples

Detritivoresand

decomposers

Crayfish

Water mold

Aquaticbacteria

Tertiary consumers

Cormorant Hawk

Secondary consumers

Fish Rodent

Primary consumers

Zooplankton Grasshopper

Producers

Phytoplankton Grass

Detritivoresand

decomposers

Earthworm

Shelf fungus

Soilbacteria

Whiteoak

Caterpillar and otherinsects on leaves

Spider

Easternchipmunk

Cedarwaxwing Beetles and

other insects

BlackberryTicks

Easterncottontail

White-tailed deer Rat snake

Red-belliedwoodpecker

Shelf fungus

Deermouse

Grasses

Earthworm

Soil bacteria

American toad

Community DisturbanceNatural vs. Human Caused

“Natural” ChangesCatastrophic:

Drought, flood, fire, volcano, earthquake, hurricane, disease

GradualClimate change, immigration and emigration,

ecological succession, evolution

Human-caused ChangesCatastrophic:

Deforestation, overgrazing, soil erosion, pesticides, urbanization, mining, toxic releases, etc.

GradualHabitat loss, introduced species,

overharvesting, pest control, salinization, waterlogging, compaction

Community Succession…..a series of directional, predictable, quantifiable changes that communities go through following a disturbance

• Primary succession: Pioneer species colonize a newly exposed area (lava flows, glacial retreat, dried lake bed).

• Secondary succession: The community changes following a disturbance (fire, hurricane, logging).

Early successional stages late successional stagesPioneer stageintermediate stageclimax community

Secondary terrestrial succession

Grasses,herbs, forbs

Shrubs,seedlings

Shrubs,poplar trees

Pines

Oaks,hardwoods

LE 5-7

Community Stability

• Resistance• A community that remains stable despite

disturbance is showing resistance to the disturbance.

• Resilience• A community shows resilience when it changes

in response to disturbance but later returns to its original state.

Community Organizers

Keystone speciesEcosystem EngineerFoundation Species

Keystone species…are species that have especially great impacts on other community structure and identity despite low numbers or biomass.

If keystone species are removed, communities change greatly.

A “keystone” holds an arch together.

Keystone speciesWhen the keystone sea otter is removed, sea urchins overgraze kelp and destroy the kelp forest community.

Ecosystem Engineer…a species that exerts influence on ecosystem or community by changing the physical environment

If ecosystem engineer is removed, communities change greatly.

•Termite mound in Australia•Termites play a vital role in recycling nutrients by building mounds from sub-surface soils •With the aid of bacteria which live in termite guts, they can digest otherwise indigestable cellulose from the plant matter that they eat

Foundation species

…a species that strongly influences community structure by creating environments suitable for other species and by major effects on ecosystem processesIf a foundation species is removed, communities change greatly.

Example: dominant forest tree such as Ponderosa Pine

What have community ecologists learned?

When we try to pick out anything by itself we find that it is bound fast by a thousand invisible cords that cannot be broken, to everything in the universe.

John Muir, Journal, July 27, 1869